Instruments for measuring impedance values desirably have the ability of making the required measurements when the device under test is powered down, i.e. with the measuring instrument providing whatever power is necessary for the measurement. However, devices which may be tested represent a wide range of voltage requirements and limitations. For instance, if pn junctions are included in a circuit under test, any applied voltage must be sufficiently low to prevent damage or undesirable turn on of the pn junctions. Because of the variable value of the excitation voltage required and because of a desire to avoid, if possible, switching of meter ranges, it is advantageous to measure the impedance ratiometrically, i.e. in terms of a standard impedance rather than in terms of the applied voltage.
Apparatus has been available heretofore for making ratiometric impedance measurements, but such apparatus has usually required fairly complicated and expensive circuitry in order to provide a digital output reading. Thus, a conventional bridge circuit or an electronic analog thereof can measure an impedance in terms of a second or standard impedance. Also, computer circuits are available which measure the voltage drop across an unknown impedance and a standard impedance and which then perform a calculation for the ratio between the two. This approach has a speed governed by the speed and complexity of the computer.
The desirable elimination of switching between different measurement ranges has also heretofore required some scheme to compress the measurement scale. In the past, a logarithmic amplifier has been employed, but unfortunately such an amplifier tends to be unstable, inaccurate, expensive, and occupies considerable printed circuit board area.
As the density of circuit boards has increased, a need has also arisen for test equipment capable of more rapidly measuring impedances, particularly small capacitances, between large numbers of points on each board without compromising measurement accuracy. To increase measurement speed and to reduce the bulk of multipoint impedance testing equipment, designers have eliminated mechanical switches for connecting test point probes with impedance measurement apparatus and have substituted high speed, high density, integrated solid state switches. However, solid state switches typically have comparatively high resistance, leakage, and capacitance which can introduce significant measurement error. Small value capacitance (&lt;0.01 microfarads) measurement is particularly troublesome due to stray capacitance in interconnecting switching networks and in some part due to the capacitance of long cables connecting test stations to remote mainframes containing measurement equipment.
What is needed is a method and apparatus for high speed measurement of impedances, particularly small capacitances, between many points on a circuit board wherein the effects of inherent capacitance of the measurement apparatus are minimized.